Perplexed by this incomprehensible equation, which surely must be based on something. Perhaps this is what it means (but needs 7 corrections) 
Assume we are concentrating on the daily new infections rather than total number
Then need to multiply by R rather than add
R is a number (mean number of infections arising from an infected person) rather than a rate (change over time)
Am not at all an epidemiologist but this takes the two measures we are given daily and combines to give what seems a sensible statistic (expected number of new cases arising from those currently appearing)
With R=0.5-0.9 and number of new cases at 3,923 this gives 1,961-3,530. However Mr Johnston stated the current value is below 4. Many measures (acidity, earthquakes) are given on a logarithmic scale. The logarithm of 3,530 cases is 3.54
This uses equation to give the right position on the scale. Unfortunately it needs rescaling. An Alert level of 1 is actually 10 cases not eradicated. Similarly the value at which the NHS is swamped is surely below 5 (100,000 daily cases).
The daily briefing and statistic provided focuses on number of recorded cases rather than actual. This still works perfectly well as an index if the measure has been obtained with consistent methods (hasn’t been) or is done so the future (unlikely)
Which leads to the conclusion to stay alert, but not sure what that means. Our suggestion is the creation of biosecurity plans, based on guidance that has incorporated the evidence and considered the full range of options for implementing. osf.io/ca5rh
